Glycolic acid (HOCH2COOH; CAS Registry Number is 79-14-1) is the simplest member of the α-hydroxy acid family of carboxylic acids. Its properties make it ideal for a broad spectrum of consumer and industrial applications, including use in water well rehabilitation, the leather industry, the oil and gas industry, the laundry and textile industry, as a monomer in the preparation of polyglycolic acid (PGA), and as a component in personal care products. Glycolic acid also is a principle ingredient for cleaners in a variety of industries (dairy and food processing equipment cleaners, household and institutional cleaners, industrial cleaners [for transportation equipment, masonry, printed circuit boards, stainless steel boiler and process equipment, cooling tower/heat exchangers], and metals processing [for metal pickling, copper brightening, etching, electroplating, electropolishing]). Recently, it has been reported that polyglycolic acid is useful as a gas barrier material (i.e., exhibits high oxygen barrier characteristics) for packing foods and carbonated drinks (WO 2005/106005 A1). However, traditional chemical synthesis of glycolic acid produces a significant amount of impurities that must be removed prior to use in preparing polyglycolic acid for gas barrier materials. New technology to commercially produce glycolic acid, especially one that produces glycolic acid in high purity and at low cost, would be eagerly received by industry.
Microbial catalysts can hydrolyze a nitrile (e.g., glycolonitrile) directly to the corresponding carboxylic acids (e.g., glycolic acid) using a nitrilase (EC 3.5.5.7), where there is no intermediate production of the corresponding amide (Equation 1), or by a combination of nitrile hydratase (EC 4.2.1.84) and amidase (EC 3.5.1.4) enzymes, where a nitrile hydratase (NHase) initially converts a nitrile to an amide, and then the amide is subsequently converted by the amidase to the corresponding carboxylic acid (Equation 2):

Enzymatic synthesis of glycolic acid requires a substantially pure form of glycolonitrile. Methods to synthesize glycolonitrile by reacting aqueous solutions of formaldehyde and hydrogen cyanide have previously been reported (U.S. Pat. Nos. 2,175,805; 2,890,238; and 5,187,301; Equation 3).HCN+HCHO→HOCH2CN  (3)
However, these methods typically result in an aqueous glycolonitrile reaction product that requires significant purification (e.g., distillative purification) as many of the impurities and/or byproducts of the reaction (including excess reactive formaldehyde) may interfere with the enzymatic conversion of glycolonitrile to glycolic acid, including catalyst inactivation. Inactivation of the enzyme catalyst decreases the overall productivity of the catalyst (i.e., total grams of glycolic acid formed per gram of catalyst), adding a significant cost to the overall process, which may make enzymatic production methods economically non-viable when compared to chemical methods of production. As such, reaction conditions that yield glycolonitrile with fewer impurities are needed, especially those that address the amount of free formaldehyde in the reaction product. The glycolonitrile synthesis conditions should 1) increase overall glycolonitrile yield, 2) minimize unwanted impurities and/or byproducts, and 3) decrease the cost to make a glycolonitrile preparation suitable for enzymatic synthesis.
Various methods are known for preparing α-hydroxy acids using the corresponding α-hydroxy nitrile as the starting material and a microorganism as the catalyst. Examples of α-hydroxy acids produced include: glycolic acid, lactic acid, 2-hydroxyisobutyric acid, 2-hydroxy-2-phenyl propionic acid, mandelic acid, 2-hydroxy-3,3-dimethyl-4-butyrolactone, and 4-methylthiobutyric acid. These products are synthesized using microorganisms, such as those belonging to the genera Nocardia, Bacillus, Brevibacterium, Aureobacterium, Pseudomonas, Caseobacter, Alcaligenes, Acinetobacter, Enterobacter, Arthrobacter, Escherichia, Micrococcus, Streptomyces, Flavobacterium, Aeromonas, Mycoplana, Cellulomonas, Erwinia, Candida, Bacteridium, Aspergillus, Penicillium, Cochliobolus, Fusarium, Rhodopseudomonas, Rhodococcus, Corynebacterium, Microbacterium, Obsumbacterium and Gordona. (JP-A-4-99495, JP-A-4-99496 and JP-A-4-218385 corresponding to U.S. Pat. No. 5,223,416; JP-A-4-99497 corresponding to U.S. Pat. No. 5,234,826; JP-A-5-95795 corresponding to U.S. Pat. No. 5,296,373; JP-A-5-21987; JP-A-5-192189 corresponding to U.S. Pat. No. 5,326,702; JP-A-6-237789 corresponding to EP-A-0610048; JP-A-6-284899 corresponding to EP-A-0610049; JP-A-7-213296 corresponding to U.S. Pat. No. 5,508,181).
However, most known methods for preparing α-hydroxy acids from the corresponding α-hydroxy nitriles as mentioned above do not produce and accumulate a product at a sufficiently high concentration to meet commercial needs. This is frequently a result of enzyme inactivation early in the reaction period. U.S. Pat. No. 5,756,306 teaches that “When an α-hydroxy nitrile is enzymatically hydrolyzed or hydrated using nitrilase or nitrile hydratase to produce an α-hydroxy acid or α-hydroxy amide, a problem occurs in that the enzyme is inactivated within a short period of time. It is therefore difficult to obtain the α-hydroxy acid or α-hydroxy amide in high concentration and high yield.” (col. 1, lines 49-54). Maintaining the aldehyde concentration (formed by the disassociation of α-hydroxy nitrile to aldehyde and hydrogen cyanide) and/or the α-hydroxy nitrile concentration in the reaction mixture within a specified range is one method to avoid this problem.
U.S. Pat. No. 5,508,181 addresses further difficulties relating to rapid enzyme inactivation. Specifically, U.S. Pat. No. 5,508,181 mentions that α-hydroxy nitrile compounds partially disassociate into the corresponding aldehydes, according to the disassociation equilibrium. These aldehydes inactivate the enzyme within a short period of time by binding to the protein, thus making it difficult to obtain α-hydroxy acid or α-hydroxy amide in a high concentration with high productivity from α-hydroxy nitriles (col. 2, lines 16-29). As a solution to prevent enzyme inactivation due to accumulation of aldehydes, phosphate or hypophosphite ions were added to the reaction mixture. U.S. Pat. No. 5,326,702 uses sulfite, disulfite, or dithionite ions to sequester aldehyde and prevent enzyme inactivation. However, the concentration of α-hydroxy acid produced and accumulated even by using such additives as described above is not great.
U.S. Pat. No. 6,037,155 teaches that low accumulation of α-hydroxy acid product is related to enzyme inactivation within a short time due to the disassociated-aldehyde accumulation. These inventors suggest that enzymatic activity is inhibited in the presence of hydrogen cyanide (Asano et al., Agricultural Biological Chemistry, Vol. 46, pages 1165-1174 (1982)) generated in the partial disassociation of the α-hydroxy nitrile in water together with the corresponding aldehyde or ketone (Mowry, David T., Chemical Reviews, Vol. 42, pages 189-283 (1948)). The inventors solved the problem of aldehyde-induced enzyme inactivation by using microorganisms whose enzyme activity could be improved by adding a cyanide substance to the reaction mixture. The addition of a cyanide substance limited the disassociation of α-hydroxy nitrile to aldehyde and hydrogen cyanide.
With specific respect to the production of glycolic acid, glycolonitrile is known to reversibly disassociate to hydrogen cyanide and formaldehyde, either of which may be involved in enzyme inactivation. U.S. Pat. No. 3,940,316 describes a process for preparing an organic acid from the corresponding nitrile using bacteria with “nitrilasic” activity, and lists glycolonitrile as a substrate. In particular, this patent describes the use of Bacillus, Bacteridium, Micrococcus, and Brevibacterium for this purpose. Though described as having nitrilasic activity, Brevibacterium R312 is the only strain used in all of the U.S. Pat. No. 3,940,316 examples. Brevibacterium R312 is known to have nitrile hydratase and amidase activities, but no nitrilase activity (Tourneix et al., Antonie van Leeuwenhoek, 52:173-182 (1986)).
A method for preparing lactic acid, glycolic acid, and 2-hydroxyisobutyric acid by using a microorganism belonging to Corynebacterium spp. is disclosed in Japanese Patent Laid-open No. Sho 61-56086. JP 09028390 discloses a method for manufacturing glycolic acid from glycolonitrile by the action of Rhodococcus or Gordona hydrolase. Selectivity for glycolic acid is reported as almost 100%, without formation of glycolic acid amide. U.S. Pat. No. 6,037,155 discloses examples of methods for producing α-hydroxy acids from α-hydroxy nitriles, including glycolic acid. This disclosure acknowledges that not all microbial catalysts can produce high concentrations of glycolic acid due to the aforementioned problems and instructs that screening studies must be conducted in order to find industrially advantageous microorganisms. U.S. Pat. No. 6,037,155 specifically identifies Variovorax spp. and Arthrobacter spp. microorganisms that are resistant to the suppressing effect of α-hydroxy nitrile or α-hydroxy acid, have durable activity, and can produce the desired product at high concentration.
Acidovorax facilis 72W (ATCC 55746) is characterized by aliphatic nitrilase (EC 3.5.5.7) activity, as well as a combination of nitrile hydratase (EC 4.2.1.84) and amidase (EC 3.5.1.4) activities. The gene encoding the A. facilis 72W (ATCC 55746) nitrilase has been cloned and recombinantly expressed (WO 01/75077 corresponding to U.S. Pat. No. 6,870,038) and Chauhan et al., Appl Microbiol Biotechnol, 61:118-122 (2003)). The A. facilis 72W nitrilase converts α-hydroxynitriles to the corresponding α-hydroxycarboxylic acids in high yield (U.S. Pat. No. 6,383,786), including glycolic acid (U.S. Pat. No. 6,416,980). However, enzyme catalysts having improved nitrilase activity relative to the A. facilis 72W nitrilase when converting glycolonitrile to glycolic acid in high yield at up to 100% conversion would be very useful in reducing the cost of manufacturing glycolic acid.
A process to economically produce glycolic acid using an enzyme catalyst requires 1) a source of high purity glycolonitrile, 2) the use of an enzyme catalyst that can convert glycolonitrile to glycolic acid in high concentrations with high purity, and 3) a method recovering the glycolic acid produced. In one embodiment, the process includes use of an enzyme catalyst having high catalyst productivity (kg glycolic acid/kg enzyme catalyst) and volumetric productivity (grams of glycolic acid/L/h). The enzyme catalyst may be employed in multiple consecutive batch reactions, or in a continuous reaction that employs constant addition of glycolonitrile and removal of glycolic acid; in either mode of operation, the catalyst activity and lifetime should be such that a high volumetric productivity and catalyst productivity are obtained, and in the case of batch reactions, the catalyst must be utilized in multiple reaction cycles without significant loss in enzyme activity between consecutive batch reactions. Nitrilases having improved activity for glycolonitrile hydrolysis can provide improvements in volumetric productivity. Given the fact that the inactivating effect of free formaldehyde (and possibly other impurities) in the glycolonitrile reaction mixture will negatively affect all nitrilase catalysts to varying extents, improvements that stabilize enzyme activity under reaction conditions for hydrolysis of glycolonitrile (resulting in a relative increase in catalyst productivity) are also needed.
Enzymatic conversion of glycolonitrile to glycolic acid using an enzyme catalyst normally results in the production of an aqueous solution comprising mostly ammonium glycolate (i.e., reactions are typically run at a pH of about 6 to about 9). Various methods can be used to obtain glycolic acid from aqueous solutions of ammonium glycolate including, but not limited to ion exchange (anionic and/or cationic), electrodialysis, reactive solvent extraction, polymerization, thermal decomposition (salt cracking), alcoholysis, and combinations thereof.
The problem to be solved is to provide a process to produce glycolic acid (in the form of the salt of acid) in high yield and with high purity. In one embodiment, the desired process should include 1) preparation of an aqueous solution comprising glycolonitrile from formaldehyde and hydrogen cyanide suitable for enzymatic conversion to ammonium glycolate (i.e., “high purity” glycolonitrile), 2) use of an enzyme catalyst having nitrilase activity the hydrolyzes the high purity glycolonitrile into ammonium glycolate, and 3) a method to obtain high purity glycolic acid from the ammonium glycolate. In another embodiment, the process includes use of an enzyme catalyst having improved nitrilase activity (thereby increasing volumetric productivity) relative to the nitrilase activity of the Acidovorax facilis 72W nitrilase, and reaction conditions that improve catalyst stability and productivity.